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Hydrothermal synthesis and magneto-optical properties of Ni-doped ZnO hexagonal columns
Journal of Magnetism and Magnetic Materials ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Hydrothermal synthesis and magneto-optical properties of Ni-doped ZnO hexagonal columns Xingyan Xu, Chuanbao Cao n Research Center of Materials Science, Department of Materials Science & Engineering, Beijing Institute of Technology, Beijing 100081, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 5 May 2014 Received in revised form 30 August 2014 Accepted 27 October 2014
Single crystal Zn1 xNixO (x ¼0, 0.02, 0.04, 0.06) hexagonal columns have been synthesized by a simple hydrothermal route. The hexagonal columns of the products are about 3 μm in diameter and about 2 μm in thickness. X-ray diffraction (XRD), Ni K-edge XANES spectra and TEM indicate that the as-prepared samples are single-crystalline wurtzite structure and no metallic Ni or other secondary phases are found in the hexagonal columns. Optical absorption and Raman results further confirm the incorporation of Ni2 þ ions in the ZnO lattice. Magnetic measurements indicate that the Zn1 xNixO hexagonal columns exhibited obvious ferromagnetic characteristic at room temperature. The coercive fields (Hc) were obtained to be 135.3, 327.79 and 127.29 Oe for x ¼ 0.02, 0.04 and 0.06, respectively. The ferromagnetism was assumed to originate from the exchange interaction between free carriers (holes or electrons) from the valence band and the localized d spins on the Ni ions. & 2014 Published by Elsevier B.V.
Keywords: Diluted magnetic semiconductors Optical properties Hexagonal columns Magnetic properties
1. Introduction In recent years, transition metal (TM) doped ZnO diluted magnetic semiconductors have attracted considerable attention for the potential application in spintronic devices because its Curie temperature is theoretically predicted to be well above room temperature [1, 2] and room temperature ferromagnetism (RTFM) has been observed experimentally in Co-, Fe-, Mn-, and Cu-doped ZnO systems [3–7]. For practical applications, an ideal DMS should exhibit ferromagnetic features at or above room temperature and have a homogenous distribution of the transition metal (TM) dopants. A considerable research effort has recently been focused on studying TM doped one-dimensional (1D) ZnO nano/microstructures (such as wires, rods, and tubes) due to their potential use in producing nano/microscale spintronic devices [8]. Among the transition metal ions, Ni is the most efficient doping elements to improve and tune the optical, electrical and magnetic properties of ZnO materials [9]. However, Ni is very much unstable metal in the ZnO matrix, its preparation is particularly challenging due to the large driving force for phase segregation into NiO and ZnO [10]. There are a range of experimental studies focusing on Nidoped ZnO where diverse magnetic properties have been observed. El-Hilo et al. [11] reported that Ni doped powders with a low Ni/Zn atomic ratio exhibit superparamagnetic behavior while n
Corresponding author. Fax: þ 86 10 68913937. . E-mail address: [email protected] (C. Cao).
the sample of 22.5% Ni shows clear ferromagnetism at room temperature. Ferromagnetism was observed in ZnO films doped 1%, 3%, and 5% of Ni [12]. The Ni doped ZnO nanocrystals also exhibited room temperature ferromagnetic behaviors [13]. There are further reports that ferromagnetism up to 350 K is also attained in Ni-doped ZnO quantum dots [14]. On the other hand, Zhou et al. [15] believe that crystalline Ni nanoparticles are the origin of ferromagnetism in Ni implanted ZnO crystals. Colloidal Ni2 þ :ZnO nanocrystals are paramagnetic, while their aggregation gives rise to robust ferromagnetism [16]. These controversial results between research groups suggest that the magnetic properties of diluted magnetic semiconductors (DMS) seem to be very sensitive to the preparation method and the structure of materials. To date, studies on Ni-doped ZnO have involved the synthesis of thin films [17], nanocrystals [13], and one dimensional structures [18]. However, few studies on Ni-doped ZnO micro-structures, especially large single crystal, were reported [19, 20]. It is well known that the properties of the materials are dependent on their size/shape, which would result in a wide range of electrical, optical, or magnetic properties and open a new domain of theoretical and technological significance. Therefore, morphology-controllable synthesis of inorganic crystals is one of the key challenges for the practical applications of these materials. Compared to nano-size crystal, large facet morphology crystal has better smooth surface and less surface defects. If it exhibits room temperature ferromagnetism, which is better to prove that ferromagnetism is an intrinsic property of doped ZnO, due to
http://dx.doi.org/10.1016/j.jmmm.2014.10.130 0304-8853/& 2014 Published by Elsevier B.V.
Please cite this article as: X. Xu, C. Cao, Journal of Magnetism and Magnetic Materials (2014), http://dx.doi.org/10.1016/j. jmmm.2014.10.130i
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67 secondary phase. Fig. 1b is enlarged XRD patterns in 30–38° re68 gion. It can be seen that with increasing concentration of Ni, the 69 peak position of Zn1 xNixO are shifted slightly to lower angles. 70 These phenomena presumably result from the larger ionic radius of Ni2 þ than that of Zn2 þ (rNi2 þ ¼0.69 Å; rZn2 þ ¼0.60 Å) [21, 22]. 71 The expansion of the lattice constants imply that Ni2 þ in72 corporates into the ZnO lattice and substitutes the Zn2 þ site. In 73 order to characterize the oxidation state and local structure of Ni 74 in ZnO lattice, the near-edge X-ray absorption fine structure 75 (NEXAFS) and extended X-ray absorption fine structure (EXAFS) 76 measurement was performed for Zn1 xNixO(x¼0.02, 0.04, 0.06). 77 Fig. 2a shows the NEXAFS spectra of Ni in the sample. The re78 2. Experimental ference spectra of Ni metal and Ni oxide (NiO and Ni2O3) are also 79 presented in Fig. 2a. The peak position and the line shape of 80 Ni-doped ZnO hexagonal columns were synthesized through a spectrum for Zn1 xNixO (x ¼0.04, 0.06) are similar to those of 81 low-temperature hydrothermal process using PVA and water as spectrum for NiO, indicating that Ni ions are divalent, which fur82 solvent. 2 mmol zinc acetate dehydrate (Zn(CH3COO)2 H2O) and ther confirms that no metallic Ni nanoclusters and other second 83 the required amount of nickel acetate dehydrate phases appeared. Fig. 2(b) compares the Fourier transform am84 (Ni(CH3COO)2 H2O) were dissolved in a PVA aqueous solution. plitude of the pure ZnO powder, as obtained from the Zn K-edge 85 The mixed aqueous solution was stirred using a magnetic stirrer at EXAFS data, with that of Zn1 xNixO columns from the Ni K-edge 86 a temperature of about 80 °C in order to make the ligands on PVA spectrum. The first and second major peaks in the radial dis87 chains fully complex with the zinc ions and nickel ions. The pH tribution function (RDF) of the ZnO powders, respectively, corre88 value of the solution was adjusted to 7.5 by adding a concentrated spond to the nearest oxygen and Zn atoms from the central Zn 89 ammonia solution. Then the solution was transferred to a Teflonatom. On the other hand, the first and second major peaks of the 90 lined stainless steel autoclave and heated at 160 °C for 10 h. The Zn1 xNixO columns at the Ni K-edge correspond to the nearest 91 morphology of the structures was controlled by adjusting the oxygen and Ni (or Zn) atoms from the central Ni atom, respec92 amounts of PVA. A white precipitate was collected by centrifugatively. Fig. 2(b) indicates that the RDF results of the samples are 93 tion and thoroughly washed with distilled water and ethanol, and very similar to each other. Thus, the EXAFS results, in conjunction finally dried at 120 °C for 10 h in vacuum. 94 with the XRD results, clearly indicate that Ni atoms atomically The structure of Zn1 xNixO hexagonal columns were char95 substitute for Zn atoms, without forming Ni metal clusters or Ni96 acterized by X-ray diffraction analysis (XRD, X′Pert Pro MPD) with containing oxide precipitates. 97 CuKα radiation (λ ¼ 0.15406 nm) and XAFS. The XAFS measureFig. 3 shows SEM images of Zn0.96Ni0.04O synthesized with 98 ment was performed on the U7C beamline of National Synchrodifferent amount of PVA. As shown in Fig. 3a, when the reaction is 99 tron Radiation Laboratory of China. The optical absorption spectra carried out in the low amount of PVA, hexagonal columns with 100 were measured in the range 200–800 nm using a UV–vis absorpdifferent sizes and thickness was observed. With increasing the 101 tion spectrums (U-4100, Hitachi). Micro-Raman scattering examount of PVA, the hexagonal columns gradually become uniform. 102 periments were done using a micro-Raman spectrometer with the When the amount of PVA was increased to 4.0 wt%, hexagonal 103 514.5 nm line of an Ar þ laser as excitation source(JY-T64000, columns with uniform size and well-defined shape were observed. 104 France). The magnetic properties were examined at room temThe hexagonal columns of the products are about 3 μm in dia105 perature using a VSM (VSM 7403, Lake Shore, USA). meter and about 2 μm in thickness. A further increasing the 106 amount of PVA up to10%, the thickness of the products sig107 nificantly increase and some bilayer hexagonal columns was ob- Q2108 3. Results and discussion 109 served in the product, as shown in Fig. 3d. TEM image shows the regular hexagonal of Zn0.96Ni0.04O (Fig. 4a). Shown in Fig. 4b, the 110 The XRD patterns of Zn1 xNixO (x ¼0, 0.02, 0.04, 0.06) samples 111 corresponding selected area electron diffraction (SAED) pattern shown in Fig. 1a, which clearly reveals that all the observed difreveal that the hexagonal columns are highly crystalline, showing 112 fraction peaks in the XRD pattern of Zn1 xNixO can be indexed to 113 their wurtzite structure in agreement with the XRD results. the ZnO wurtzite structure (JCPDS no. 79-2205) without forming 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 Fig. 1. (a) XRD patterns of Zn1 xNixO(x ¼0, 0.02, 0.04, 0.06) hexagonal columns. 132 reducing defects of surfaces and crystal boundaries influence to the magnetic properties. Therefore, in this paper, we fabricated single crystal Zn1 xNixO (x¼ 0, 0.02, 0.04, 0.06) hexagonal columns by a simple hydrothermal method. Zn1 xNixO hexagonal columns exhibit obvious room temperature ferromagnetism, which is more credible than that of doped ZnO nanomaterials. The origin of the observed room temperature ferromagnetism is most likely due to the exchange interaction between local-spin polarized electrons and conduction electrons.
Please cite this article as: X. Xu, C. Cao, Journal of Magnetism and Magnetic Materials (2014), http://dx.doi.org/10.1016/j. jmmm.2014.10.130i
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Fig. 2. (a) Normalized X-ray absorption coefficient for Zn1 xNixO and reference samples of Ni and NiO powders at the Ni K-edge at room temperature. (b) Magnitude of Fourier-transformed XAFS from the Ni K-edge from the Ni K-edge for Zn1 xNixO and the Zn K-edge for pure ZnO.
The layer-by-layer growth manner of the Zn1 xNixO hexagonal columns can be detected from SEM photograph of the columns in the sample. In Fig. 3c, the typical stages of the growth manner marked by 1, 2, 3, 4. The growth manner is illustrated in Fig. 5. In Fig. 5a and b, small circular area appears on the top of an individual Zn1 xNixO hexagonal column, suggesting that the Zn1 xNixO hexagonal layer is at the earlier growth stage. In Fig. 5c, the circular area is enlarged and gradually became hexagonal, which is mirrored in Fig. 3c (1–4). In Fig. 5d and e, a full layer of Zn1 xNixO formed on the top of the individual column, and also another new entire layer-by-layer procreation stage again begins in Fig. 5e.
Fig. 6 shows the room temperature photoluminescence spectra of undoped ZnO and Zn1 xNixO hexagonal columns for different Ni doping concentrations. It can be seen that all the samples contain a strong ultraviolet peak at about 388 nm. The ultraviolet emission of Zn1 xNixO is generally attributed to exciton emission originating from recombination of free excitations through an excitonexciton collision process [23]. A broad band observed between 400 and 500 nm might be attributed to the intrinsic defects in the doped ZnO material. However, in these Ni-doped ZnO samples, the green emission band was not observed. The peak centered at about 420 nm originates from the coordinative, unsaturated Zn sites in the ZnO hexagonal columns, which is often induced by the
Fig. 3. SEM images of Zn0.96Ni0.04O with different added amounts of PVP: (a) 1%, (b) 2%, (c) 4% and (d) 10%.
Please cite this article as: X. Xu, C. Cao, Journal of Magnetism and Magnetic Materials (2014), http://dx.doi.org/10.1016/j. jmmm.2014.10.130i
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Fig. 4. (a) TEM and (b) SAED pattern for Zn0.96Ni0.04O prepared with 4% amounts of PVP.
Fig. 5. Layer-by-layer procreation manner of an individual Zn1 xNixO hexagonal column.
of Zn0.96Ni0.04O and then influence ferromagnetism of Zn0.96Ni0.04O. The exact mechanism responsible for the emission is still in the study. In order to investigate the influence of Ni doping on microstructure and vibrational properties, Raman scattering experiments were carried out. Fig. 7 shows the room-temperature Raman spectra ranging from 250 to 1400 cm 1. The sharpest and strongest peak at about 437 cm 1 can be assigned to the nonpolar optical phonon E2(high) mode of ZnO, which is the strongest mode in wurtzite crystal structure. The peaks at 330 and 379 cm 1 are assigned to the second-order vibration mode and the A1(TO)
Fig. 6. Room temperature photoluminescence spectra of the Zn1 xNixO (x ¼0, 0.02, 0.04, 0.06).
introduction of impurity into the semiconductor [24], due to the substitution of Ni2 þ ions for the Zn2 þ ions resulting from the Ni incorporation. Compared to the peaks of Zn0.98Ni0.02O and Zn0.94Ni0.06O at 420 nm, a shift is observed in the PL spectra of Zn0.96Ni0.04O. In addition, the blue emission at 458 nm of Zn0.96Ni0.04O may come from the electron transition from the Zn interstitial level to the top of the valence bands. Ni behaved as a donor with a shallow level below the conduction band. The positions of the blue emission peaks are very close to the recent results obtained by Peng et al. [25], Wu et al. [26], and Wang and Gao [27]. The shift and additional peak may be related to internal structure
Fig. 7. Room-temperature Raman spectra of the Zn1 xNixO (x ¼0.02, 0.04, 0.06) hexagonal columns.
Please cite this article as: X. Xu, C. Cao, Journal of Magnetism and Magnetic Materials (2014), http://dx.doi.org/10.1016/j. jmmm.2014.10.130i
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conduction electrons. Subsequently, the spin polarization conductive electrons perform an exchange interaction with spin-polarized electrons of other Ni ions. Thus, after the long-range exchange interaction, almost all Ni moments align in the same direction. The conductive electrons are regarded as media to contact all Ni ions. As a result, the material exhibits ferromagnetism. These results strongly indicate that the ferromagnetism of Zn1 xNixO columns is induced by doped Ni ions and is an intrinsic property of the columns.
4. Conclusions
Fig. 8. Magnetization of Zn1 xNixO (x ¼0, 0.02, 0.04, 0.06) as a function of applied field measured at room temperature.
mode, respectively. In addition, in the high-wavenumber region, the broad peak at 1150 cm 1 is deconvoluted in two peaks, as shown in inset of Fig. 7. The peak at 1104 cm 1 can be attributed to 2LO and the peak at 1158 cm 1 contains contributions of 2A1(LO) and 2E1(LO) modes [28]. There are no apparent impurityrelated modes in all the Raman spectrum of the Zn1 xNixO. The Raman scattering results are well consistent with the above results. The magnetic property of the as-synthesized Zn1 xNixO (x ¼0, 0.02, 0.04, 0.06 ) hexagonal columns was investigated with VSM at room temperature. Fig. 8 shows the magnetic hysteresis (M–H) loops of the Zn1 xNixO hexagonal columns with different Ni2 þ contents. The pure ZnO sample has a diamagnetic behavior. The M–H loops for the Zn1 xNixO(x ¼0.02, 0.04, 0.06) samples exhibit the coercivities field (Hc) and saturation magnetization (Ms) of 135.3, 327.79, 127.29 Oe and 0.0018, 0.0037, 0.00327 emu/g, respectively, which indicates the ferromagnetic nature of these Zn1 xNixO hexagonal columns at room temperature. It is observed that with increasing Ni2 þ content, magnetic properties firstly increases and then decreases. As the Ni concentration increases, Ni atoms come close to each other making the Ni2 þ –Ni2 þ nearest neighbors. The superexchange interactions between these neighboring Ni2 þ ions are antiferromagnetic. Therefore, increasing Ni concentration will increase the volume fraction of Ni2 þ ions with Ni2 þ nearest neighbors. As a result, the enhanced antiferromagnetic interaction suppresses ferromagnetic coupling. As to the origin of ferromagnetic behavior observed in Ni doped ZnO hexagonal columns, there are a few of controversial explanations, one of which is the formation of some Ni-related secondary phase, such as metallic Ni clusters or Ni-oxide precipitates. First, the origin from NiO can be easily ruled out, since bulk NiO is antiferromagnetism (TN ¼523 K) at room temperature. The other possible secondary phasesNi2O3 are paramagnetic materials [29]. Secondly, the existence of metallic Ni is also an unlikely source of this ferromagnetism because the synthesis of Ni doped ZnO samples is performed in water as well as OH , which can prevent the formation of metallic Ni nanoclusters to some extent. In addition, XRD, EXAFS, absorption and Raman spectra results clearly show that there is no metallic Ni clusters or Nioxide precipitates in the samples. Hence the substitution of Ni2 þ in the place of Zn2 þ in ZnO is the key mechanism for the room temperature ferromagnetism. According to Rudderman–Kittel– Yoshida (RKKY) theory [30], the magnetism is due to the exchange interaction between local-spin polarized electrons and conduction electrons. This interaction leads to the spin polarizations of
In summary, we have synthesized the single crystal Zn1 xNixO (x ¼0.02, 0.04, 0.06) hexagonal columns though simple hydrothermal method. XRD, Ni K-edge XANES spectra, Raman spectra and PL results demonstrate that Zn1 xNixO columns were in a pure wurtzite structure and Ni2 þ ions had substituted Zn2 þ sites. The magnetic hysteresis loops of the Zn1 xNixO showed obvious room-temperature ferromagnetic characteristic and the coercivity (Hc) are 135.3, 327.79 and 127.29 Oe for x ¼0.02, 0.04 and 0.06, respectively. The origin of the observed room temperature ferromagnetism is most likely due to the exchange interaction between local-spin polarized electrons and conduction electrons. The ferromagnetism of the synthesized Zn1 xNixO hexagonal columns makes them potentially useful as build components for spintronic devices.
Acknowledgment This work was supported by National Natural Science Foundation of China (Grants 50972017 and 20471007).
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Hydrothermal synthesis and magneto-optical properties of Ni-doped ZnO hexagonal columns Xingyan Xu, Chuanbao Cao n Research Center of Materials Science, Department of Materials Science & Engineering, Beijing Institute of Technology, Beijing 100081, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 5 May 2014 Received in revised form 30 August 2014 Accepted 27 October 2014
Single crystal Zn1 xNixO (x ¼0, 0.02, 0.04, 0.06) hexagonal columns have been synthesized by a simple hydrothermal route. The hexagonal columns of the products are about 3 μm in diameter and about 2 μm in thickness. X-ray diffraction (XRD), Ni K-edge XANES spectra and TEM indicate that the as-prepared samples are single-crystalline wurtzite structure and no metallic Ni or other secondary phases are found in the hexagonal columns. Optical absorption and Raman results further confirm the incorporation of Ni2 þ ions in the ZnO lattice. Magnetic measurements indicate that the Zn1 xNixO hexagonal columns exhibited obvious ferromagnetic characteristic at room temperature. The coercive fields (Hc) were obtained to be 135.3, 327.79 and 127.29 Oe for x ¼ 0.02, 0.04 and 0.06, respectively. The ferromagnetism was assumed to originate from the exchange interaction between free carriers (holes or electrons) from the valence band and the localized d spins on the Ni ions. & 2014 Published by Elsevier B.V.
Keywords: Diluted magnetic semiconductors Optical properties Hexagonal columns Magnetic properties
1. Introduction In recent years, transition metal (TM) doped ZnO diluted magnetic semiconductors have attracted considerable attention for the potential application in spintronic devices because its Curie temperature is theoretically predicted to be well above room temperature [1, 2] and room temperature ferromagnetism (RTFM) has been observed experimentally in Co-, Fe-, Mn-, and Cu-doped ZnO systems [3–7]. For practical applications, an ideal DMS should exhibit ferromagnetic features at or above room temperature and have a homogenous distribution of the transition metal (TM) dopants. A considerable research effort has recently been focused on studying TM doped one-dimensional (1D) ZnO nano/microstructures (such as wires, rods, and tubes) due to their potential use in producing nano/microscale spintronic devices [8]. Among the transition metal ions, Ni is the most efficient doping elements to improve and tune the optical, electrical and magnetic properties of ZnO materials [9]. However, Ni is very much unstable metal in the ZnO matrix, its preparation is particularly challenging due to the large driving force for phase segregation into NiO and ZnO [10]. There are a range of experimental studies focusing on Nidoped ZnO where diverse magnetic properties have been observed. El-Hilo et al. [11] reported that Ni doped powders with a low Ni/Zn atomic ratio exhibit superparamagnetic behavior while n
Corresponding author. Fax: þ 86 10 68913937. . E-mail address: [email protected] (C. Cao).
the sample of 22.5% Ni shows clear ferromagnetism at room temperature. Ferromagnetism was observed in ZnO films doped 1%, 3%, and 5% of Ni [12]. The Ni doped ZnO nanocrystals also exhibited room temperature ferromagnetic behaviors [13]. There are further reports that ferromagnetism up to 350 K is also attained in Ni-doped ZnO quantum dots [14]. On the other hand, Zhou et al. [15] believe that crystalline Ni nanoparticles are the origin of ferromagnetism in Ni implanted ZnO crystals. Colloidal Ni2 þ :ZnO nanocrystals are paramagnetic, while their aggregation gives rise to robust ferromagnetism [16]. These controversial results between research groups suggest that the magnetic properties of diluted magnetic semiconductors (DMS) seem to be very sensitive to the preparation method and the structure of materials. To date, studies on Ni-doped ZnO have involved the synthesis of thin films [17], nanocrystals [13], and one dimensional structures [18]. However, few studies on Ni-doped ZnO micro-structures, especially large single crystal, were reported [19, 20]. It is well known that the properties of the materials are dependent on their size/shape, which would result in a wide range of electrical, optical, or magnetic properties and open a new domain of theoretical and technological significance. Therefore, morphology-controllable synthesis of inorganic crystals is one of the key challenges for the practical applications of these materials. Compared to nano-size crystal, large facet morphology crystal has better smooth surface and less surface defects. If it exhibits room temperature ferromagnetism, which is better to prove that ferromagnetism is an intrinsic property of doped ZnO, due to
http://dx.doi.org/10.1016/j.jmmm.2014.10.130 0304-8853/& 2014 Published by Elsevier B.V.
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67 secondary phase. Fig. 1b is enlarged XRD patterns in 30–38° re68 gion. It can be seen that with increasing concentration of Ni, the 69 peak position of Zn1 xNixO are shifted slightly to lower angles. 70 These phenomena presumably result from the larger ionic radius of Ni2 þ than that of Zn2 þ (rNi2 þ ¼0.69 Å; rZn2 þ ¼0.60 Å) [21, 22]. 71 The expansion of the lattice constants imply that Ni2 þ in72 corporates into the ZnO lattice and substitutes the Zn2 þ site. In 73 order to characterize the oxidation state and local structure of Ni 74 in ZnO lattice, the near-edge X-ray absorption fine structure 75 (NEXAFS) and extended X-ray absorption fine structure (EXAFS) 76 measurement was performed for Zn1 xNixO(x¼0.02, 0.04, 0.06). 77 Fig. 2a shows the NEXAFS spectra of Ni in the sample. The re78 2. Experimental ference spectra of Ni metal and Ni oxide (NiO and Ni2O3) are also 79 presented in Fig. 2a. The peak position and the line shape of 80 Ni-doped ZnO hexagonal columns were synthesized through a spectrum for Zn1 xNixO (x ¼0.04, 0.06) are similar to those of 81 low-temperature hydrothermal process using PVA and water as spectrum for NiO, indicating that Ni ions are divalent, which fur82 solvent. 2 mmol zinc acetate dehydrate (Zn(CH3COO)2 H2O) and ther confirms that no metallic Ni nanoclusters and other second 83 the required amount of nickel acetate dehydrate phases appeared. Fig. 2(b) compares the Fourier transform am84 (Ni(CH3COO)2 H2O) were dissolved in a PVA aqueous solution. plitude of the pure ZnO powder, as obtained from the Zn K-edge 85 The mixed aqueous solution was stirred using a magnetic stirrer at EXAFS data, with that of Zn1 xNixO columns from the Ni K-edge 86 a temperature of about 80 °C in order to make the ligands on PVA spectrum. The first and second major peaks in the radial dis87 chains fully complex with the zinc ions and nickel ions. The pH tribution function (RDF) of the ZnO powders, respectively, corre88 value of the solution was adjusted to 7.5 by adding a concentrated spond to the nearest oxygen and Zn atoms from the central Zn 89 ammonia solution. Then the solution was transferred to a Teflonatom. On the other hand, the first and second major peaks of the 90 lined stainless steel autoclave and heated at 160 °C for 10 h. The Zn1 xNixO columns at the Ni K-edge correspond to the nearest 91 morphology of the structures was controlled by adjusting the oxygen and Ni (or Zn) atoms from the central Ni atom, respec92 amounts of PVA. A white precipitate was collected by centrifugatively. Fig. 2(b) indicates that the RDF results of the samples are 93 tion and thoroughly washed with distilled water and ethanol, and very similar to each other. Thus, the EXAFS results, in conjunction finally dried at 120 °C for 10 h in vacuum. 94 with the XRD results, clearly indicate that Ni atoms atomically The structure of Zn1 xNixO hexagonal columns were char95 substitute for Zn atoms, without forming Ni metal clusters or Ni96 acterized by X-ray diffraction analysis (XRD, X′Pert Pro MPD) with containing oxide precipitates. 97 CuKα radiation (λ ¼ 0.15406 nm) and XAFS. The XAFS measureFig. 3 shows SEM images of Zn0.96Ni0.04O synthesized with 98 ment was performed on the U7C beamline of National Synchrodifferent amount of PVA. As shown in Fig. 3a, when the reaction is 99 tron Radiation Laboratory of China. The optical absorption spectra carried out in the low amount of PVA, hexagonal columns with 100 were measured in the range 200–800 nm using a UV–vis absorpdifferent sizes and thickness was observed. With increasing the 101 tion spectrums (U-4100, Hitachi). Micro-Raman scattering examount of PVA, the hexagonal columns gradually become uniform. 102 periments were done using a micro-Raman spectrometer with the When the amount of PVA was increased to 4.0 wt%, hexagonal 103 514.5 nm line of an Ar þ laser as excitation source(JY-T64000, columns with uniform size and well-defined shape were observed. 104 France). The magnetic properties were examined at room temThe hexagonal columns of the products are about 3 μm in dia105 perature using a VSM (VSM 7403, Lake Shore, USA). meter and about 2 μm in thickness. A further increasing the 106 amount of PVA up to10%, the thickness of the products sig107 nificantly increase and some bilayer hexagonal columns was ob- Q2108 3. Results and discussion 109 served in the product, as shown in Fig. 3d. TEM image shows the regular hexagonal of Zn0.96Ni0.04O (Fig. 4a). Shown in Fig. 4b, the 110 The XRD patterns of Zn1 xNixO (x ¼0, 0.02, 0.04, 0.06) samples 111 corresponding selected area electron diffraction (SAED) pattern shown in Fig. 1a, which clearly reveals that all the observed difreveal that the hexagonal columns are highly crystalline, showing 112 fraction peaks in the XRD pattern of Zn1 xNixO can be indexed to 113 their wurtzite structure in agreement with the XRD results. the ZnO wurtzite structure (JCPDS no. 79-2205) without forming 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 Fig. 1. (a) XRD patterns of Zn1 xNixO(x ¼0, 0.02, 0.04, 0.06) hexagonal columns. 132 reducing defects of surfaces and crystal boundaries influence to the magnetic properties. Therefore, in this paper, we fabricated single crystal Zn1 xNixO (x¼ 0, 0.02, 0.04, 0.06) hexagonal columns by a simple hydrothermal method. Zn1 xNixO hexagonal columns exhibit obvious room temperature ferromagnetism, which is more credible than that of doped ZnO nanomaterials. The origin of the observed room temperature ferromagnetism is most likely due to the exchange interaction between local-spin polarized electrons and conduction electrons.
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Fig. 2. (a) Normalized X-ray absorption coefficient for Zn1 xNixO and reference samples of Ni and NiO powders at the Ni K-edge at room temperature. (b) Magnitude of Fourier-transformed XAFS from the Ni K-edge from the Ni K-edge for Zn1 xNixO and the Zn K-edge for pure ZnO.
The layer-by-layer growth manner of the Zn1 xNixO hexagonal columns can be detected from SEM photograph of the columns in the sample. In Fig. 3c, the typical stages of the growth manner marked by 1, 2, 3, 4. The growth manner is illustrated in Fig. 5. In Fig. 5a and b, small circular area appears on the top of an individual Zn1 xNixO hexagonal column, suggesting that the Zn1 xNixO hexagonal layer is at the earlier growth stage. In Fig. 5c, the circular area is enlarged and gradually became hexagonal, which is mirrored in Fig. 3c (1–4). In Fig. 5d and e, a full layer of Zn1 xNixO formed on the top of the individual column, and also another new entire layer-by-layer procreation stage again begins in Fig. 5e.
Fig. 6 shows the room temperature photoluminescence spectra of undoped ZnO and Zn1 xNixO hexagonal columns for different Ni doping concentrations. It can be seen that all the samples contain a strong ultraviolet peak at about 388 nm. The ultraviolet emission of Zn1 xNixO is generally attributed to exciton emission originating from recombination of free excitations through an excitonexciton collision process [23]. A broad band observed between 400 and 500 nm might be attributed to the intrinsic defects in the doped ZnO material. However, in these Ni-doped ZnO samples, the green emission band was not observed. The peak centered at about 420 nm originates from the coordinative, unsaturated Zn sites in the ZnO hexagonal columns, which is often induced by the
Fig. 3. SEM images of Zn0.96Ni0.04O with different added amounts of PVP: (a) 1%, (b) 2%, (c) 4% and (d) 10%.
Please cite this article as: X. Xu, C. Cao, Journal of Magnetism and Magnetic Materials (2014), http://dx.doi.org/10.1016/j. jmmm.2014.10.130i
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Fig. 4. (a) TEM and (b) SAED pattern for Zn0.96Ni0.04O prepared with 4% amounts of PVP.
Fig. 5. Layer-by-layer procreation manner of an individual Zn1 xNixO hexagonal column.
of Zn0.96Ni0.04O and then influence ferromagnetism of Zn0.96Ni0.04O. The exact mechanism responsible for the emission is still in the study. In order to investigate the influence of Ni doping on microstructure and vibrational properties, Raman scattering experiments were carried out. Fig. 7 shows the room-temperature Raman spectra ranging from 250 to 1400 cm 1. The sharpest and strongest peak at about 437 cm 1 can be assigned to the nonpolar optical phonon E2(high) mode of ZnO, which is the strongest mode in wurtzite crystal structure. The peaks at 330 and 379 cm 1 are assigned to the second-order vibration mode and the A1(TO)
Fig. 6. Room temperature photoluminescence spectra of the Zn1 xNixO (x ¼0, 0.02, 0.04, 0.06).
introduction of impurity into the semiconductor [24], due to the substitution of Ni2 þ ions for the Zn2 þ ions resulting from the Ni incorporation. Compared to the peaks of Zn0.98Ni0.02O and Zn0.94Ni0.06O at 420 nm, a shift is observed in the PL spectra of Zn0.96Ni0.04O. In addition, the blue emission at 458 nm of Zn0.96Ni0.04O may come from the electron transition from the Zn interstitial level to the top of the valence bands. Ni behaved as a donor with a shallow level below the conduction band. The positions of the blue emission peaks are very close to the recent results obtained by Peng et al. [25], Wu et al. [26], and Wang and Gao [27]. The shift and additional peak may be related to internal structure
Fig. 7. Room-temperature Raman spectra of the Zn1 xNixO (x ¼0.02, 0.04, 0.06) hexagonal columns.
Please cite this article as: X. Xu, C. Cao, Journal of Magnetism and Magnetic Materials (2014), http://dx.doi.org/10.1016/j. jmmm.2014.10.130i
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conduction electrons. Subsequently, the spin polarization conductive electrons perform an exchange interaction with spin-polarized electrons of other Ni ions. Thus, after the long-range exchange interaction, almost all Ni moments align in the same direction. The conductive electrons are regarded as media to contact all Ni ions. As a result, the material exhibits ferromagnetism. These results strongly indicate that the ferromagnetism of Zn1 xNixO columns is induced by doped Ni ions and is an intrinsic property of the columns.
4. Conclusions
Fig. 8. Magnetization of Zn1 xNixO (x ¼0, 0.02, 0.04, 0.06) as a function of applied field measured at room temperature.
mode, respectively. In addition, in the high-wavenumber region, the broad peak at 1150 cm 1 is deconvoluted in two peaks, as shown in inset of Fig. 7. The peak at 1104 cm 1 can be attributed to 2LO and the peak at 1158 cm 1 contains contributions of 2A1(LO) and 2E1(LO) modes [28]. There are no apparent impurityrelated modes in all the Raman spectrum of the Zn1 xNixO. The Raman scattering results are well consistent with the above results. The magnetic property of the as-synthesized Zn1 xNixO (x ¼0, 0.02, 0.04, 0.06 ) hexagonal columns was investigated with VSM at room temperature. Fig. 8 shows the magnetic hysteresis (M–H) loops of the Zn1 xNixO hexagonal columns with different Ni2 þ contents. The pure ZnO sample has a diamagnetic behavior. The M–H loops for the Zn1 xNixO(x ¼0.02, 0.04, 0.06) samples exhibit the coercivities field (Hc) and saturation magnetization (Ms) of 135.3, 327.79, 127.29 Oe and 0.0018, 0.0037, 0.00327 emu/g, respectively, which indicates the ferromagnetic nature of these Zn1 xNixO hexagonal columns at room temperature. It is observed that with increasing Ni2 þ content, magnetic properties firstly increases and then decreases. As the Ni concentration increases, Ni atoms come close to each other making the Ni2 þ –Ni2 þ nearest neighbors. The superexchange interactions between these neighboring Ni2 þ ions are antiferromagnetic. Therefore, increasing Ni concentration will increase the volume fraction of Ni2 þ ions with Ni2 þ nearest neighbors. As a result, the enhanced antiferromagnetic interaction suppresses ferromagnetic coupling. As to the origin of ferromagnetic behavior observed in Ni doped ZnO hexagonal columns, there are a few of controversial explanations, one of which is the formation of some Ni-related secondary phase, such as metallic Ni clusters or Ni-oxide precipitates. First, the origin from NiO can be easily ruled out, since bulk NiO is antiferromagnetism (TN ¼523 K) at room temperature. The other possible secondary phasesNi2O3 are paramagnetic materials [29]. Secondly, the existence of metallic Ni is also an unlikely source of this ferromagnetism because the synthesis of Ni doped ZnO samples is performed in water as well as OH , which can prevent the formation of metallic Ni nanoclusters to some extent. In addition, XRD, EXAFS, absorption and Raman spectra results clearly show that there is no metallic Ni clusters or Nioxide precipitates in the samples. Hence the substitution of Ni2 þ in the place of Zn2 þ in ZnO is the key mechanism for the room temperature ferromagnetism. According to Rudderman–Kittel– Yoshida (RKKY) theory [30], the magnetism is due to the exchange interaction between local-spin polarized electrons and conduction electrons. This interaction leads to the spin polarizations of
In summary, we have synthesized the single crystal Zn1 xNixO (x ¼0.02, 0.04, 0.06) hexagonal columns though simple hydrothermal method. XRD, Ni K-edge XANES spectra, Raman spectra and PL results demonstrate that Zn1 xNixO columns were in a pure wurtzite structure and Ni2 þ ions had substituted Zn2 þ sites. The magnetic hysteresis loops of the Zn1 xNixO showed obvious room-temperature ferromagnetic characteristic and the coercivity (Hc) are 135.3, 327.79 and 127.29 Oe for x ¼0.02, 0.04 and 0.06, respectively. The origin of the observed room temperature ferromagnetism is most likely due to the exchange interaction between local-spin polarized electrons and conduction electrons. The ferromagnetism of the synthesized Zn1 xNixO hexagonal columns makes them potentially useful as build components for spintronic devices.
Acknowledgment This work was supported by National Natural Science Foundation of China (Grants 50972017 and 20471007).
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